In a speech on Tuesday to update the nation on the battle against the Islamic State, given against the backdrop of the Orlando, Fla., massacre, President Obama gave the most powerful rebuke yet to the increasingly unreasonable and dangerous ravings of the presumptive Republican presidential nominee, Donald Trump.

Mr. Obama started by listing the ways in which his administration has worked to subdue the threat of terrorism abroad and at home. And because of the use of American and allied force, the Islamic State is losing ground in Syria, Iraq and Libya, he said.

Then he addressed the accusation — a fetishized Republican talking point, repeated by Mr. Trump after Orlando — that Mr. Obama is surrendering to the enemy by avoiding the label “radical Islam.” The idea that reciting those words would help magically defeat the terrorists is absurd, and worse. It plays into the desire of groups like ISIS and Al Qaeda to make the war, as Mr. Obama said, “a war between Islam and America, or between Islam and the West. They want to claim that they are the true leaders of over a billion Muslims around the world who reject their crazy notions.”

Implying that these terrorists speak for Islam aids their propaganda, Mr. Obama said. “That’s how they recruit. And if we fall into the trap of painting all Muslims with a broad brush, and imply that we are at war with an entire religion, then we are doing the terrorists’ work for them.”

The president went on to condemn Mr. Trump and his defenders and enablers for “language that singles out immigrants and suggests entire religious communities are complicit in violence.”He warned against abandoning “the pluralism and the openness, our rule of law, our civil liberties, the very things that make this country great. The very things that make us exceptional.”

“Where does this stop?” Mr. Obama asked. “The Orlando killer, one of the San Bernardino killers, the Fort Hood killer, they were all U.S. citizens. Are we going to start treating all Muslim-Americans differently? Are we going to start subjecting them to special surveillance? Are we going to start discriminating against them, because of their faith?”

Of these ideas, front and center in the Trump campaign, Mr. Obama pointedly asked, “Do Republican officials actually agree with this?” The silence from those leaders speaks volumes.

On Monday, Mr. Trump seized the Orlando horror to summon Americans to panic, repeating his call to seal the borders to Muslims and refugees, to expand the surveillance-and-security state, to keep the nation awash in guns, to treat all Muslims as terrorists or potential terrorists or the complicit enablers of terrorism.

On Tuesday, Mr. Obama in response gave voice to what Americans of good will understand:

“We’ve gone through moments in our history before when we acted out of fear, and we came to regret it. We have seen our government mistreat our fellow citizens, and it has been a shameful part of our history.”

On Thursday, the president plans to travel to Orlando to bring solace to grieving families and a stricken city. It is all but impossible to imagine the Republicans’ leading presidential contender offering similar leadership, or having the ability to bring unity from tragedy. Which is a sign of how far the party has fallen, behind the banner of Donald Trump.